1. Field of the Invention
This invention relates to the production of an alkaline earth metal aluminide material by a carbothermic reaction involving a reducible aluminum-bearing compound, a second reducible metal compound, and a carbonaceous reducing agent.
2. Description of the Related Art
Conventionally, aluminum is prepared by the electrolytic reduction of alumina in a Hall cell. However, processes involving the direct reduction of aluminum-bearing salts or oxides at high temperatures using carbonaceous reducing agents to form either aluminum or an aluminum alloy have been explored because of their potential for energy and cost savings.
In Cochran U.S. Pat. No. 3,971,653, there is described and claimed a carbothermic process wherein Al.sub.2 O.sub.3 is reacted with carbon in a first zone to form a first liquid comprising alumina (Al.sub.2 O.sub.3) and aluminum carbide (Al.sub.4 C.sub.3). This liquid is then decomposed with increased temperature and/or decreased pressure to a second liquid comprising aluminum and carbon. Aluminum is then recovered from this second liquid.
Persson U.S. Pat. No. 4,385,930 discloses a method for producing aluminum which comprises providing a charge of carbon in a primary zone of an electric arc furnace containing a mixture of aluminum and aluminum carbide and providing a charge of alumina in a secondary zone of the furnace. The zones are heated to react aluminum with carbon to form aluminum carbide in the mixture in the first zone. The mixture flows into the second zone where the aluminum carbide reacts with alumina to form additional aluminum and an aluminum oxycarbide slag. This slag flows into the first zone for reaction with the aluminum carbide to produce aluminum which is tapped out of the second zone.
Cochran et al U.S. Pat. No. 4,053,303 describes a three step carbothermic reduction process to form aluminum-silicon alloys. Sources of alumina, silica, and carbon are reacted at a temperature of 1500.degree. to 1600.degree. C. to form silicon carbide and carbon monoxide. This mix is then brought to a temperature of 1600.degree. to 1900.degree. C. to form aluminum oxycarbide and carbon monoxide. The mix, now containing both silicon carbide and aluminum oxycarbide, is finally brought to a temperature in the range of 1950.degree. to 2200.degree. C. to produce an aluminum-silicon alloy.
The carbothermic production of an aluminum-silicon alloy is also described in European patent application publication No. 0-097,993. The aluminum-silicon alloy is produced from a mixture of oxides of aluminum and silicon and oxides of alkali or alkaline earth metals by reducing the oxides using a carbon-based reducing agent in the presence of a plasma arc burner in a shaft reactor filled with reducing material. The reduction reaction takes place at a temperature exceeding 2000.degree. C. with liquid products consisting of an aluminum-silicon alloy collected at the base of the shaft and the alkaline and/or alkaline earth metal oxides separated at the top of the reactor.
Such carbothermic processes, however, involve vaporization of some of the aluminum formed therein as well as back reaction problems that decrease the amount of yield which actually can be realized in conducting such a reduction process. To address this problem, Cochran et al U.S. Pat. No. 4,299,619 taught the production of aluminum by carbothermic reduction in a shaft-type reactor wherein an aluminum carbide precursor is formed by reacting alumina and carbon in an upper reaction zone of the reactor. The first liquid formed therein, comprising alumina and aluminum carbide, is transferred to a lower reaction zone to produce aluminum. Gaseous aluminum and Al.sub.2 O vapor, which may be formed in the lower zone, may then be reclaimed in the upper zone which is maintained at a lower temperature. Any aluminum carbide separated from the liquid in the second zone may be returned to the first zone. In this way, at least some of the problems with regard to vaporization and back reaction may be alleviated.
Another problem which has been encountered in the production of aluminum by direct carbothermic reduction is the impurity of the aluminum product recovered from the process. Kibby U.S. Pat. Nos. 4,419,126; 4,388,107; and 4,216,010 and Moore U.S. Pat. No. 4,409,021 address the problem of contamination of aluminum from a carbothermic process with aluminum carbide. The aluminum carbide-bearing aluminum is reacted with an alumina slag in the absence of reactive carbon. The reaction is said to produce aluminum and carbon monoxide or an aluminum tetraoxycarbide depending upon the reaction temperature. The patents refer to the use of a slag which also contains CaO to reduce the reaction temperature from about 2000.degree. C. (3632.degree. F.) down to about 1500.degree. C. (2732.degree. F). Two possible modes of reaction are described. A reduction mode is said to involve the reduction of alumina by aluminum carbide at 2050.degree. C. or higher to form molten aluminum and carbon monoxide. The other mode, termed the extraction mode, is said to involve the reaction between alumina and aluminum carbide to form non-metallic slag compounds, such as aluminum tetraoxycarbide.
Fijushige et al U.S. Pat. No. 4,445,934 teaches the formation of aluminum in a single step in a blast furnace using a charge containing an alumina-containing material and a mixture of a carbon material and a fluxing agent. The fluxing agent may be CaO or CaCO.sub.3 or a mineral containing calcia or magnesia.
For the direct reduction of aluminum-bearing compounds with a carbonaceous material, i.e., a carbothermic process, to be economically attractive, the process should have minimum vaporization and back reaction losses, i.e., high yield, while producing an aluminum-bearing material from which aluminum or the other metal in the material may subsequently be recovered at a lower temperature.